rust-lang / rust-lang/rust-clippy
`unnecessary_lazy_evaluations`: lint `{Option,Result}::map_or_else`
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Description
Description
None of the following currently get linted:
(EDIT: this used to include map_or, but as the comments below rightfully say, its closure can't actually be made eager)
#![allow(unused)]
#![warn(clippy::unnecessary_lazy_evaluations)]
fn main() {
let o = Some(42);
o.map_or_else(|| 42, |_| 42);
let r: Result<i32, i32> = Ok(42);
r.map_or_else(|_| 42, |_| 42);
}
It would be cool if they did:
#![allow(unused)]
#![warn(clippy::unnecessary_lazy_evaluations)]
fn main() {
let o = Some(42);
o.map_or(42, |_| 42);
let r: Result<i32, i32> = Ok(42);
r.map_or(42, |_| 42);
}
Version
rustc 1.92.0-nightly (839222065 2025-10-05)
binary: rustc
commit-hash: 839222065a44ac21c15df68ed2f2f3c0127b0b8e
commit-date: 2025-10-05
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.2
Additional Labels
@rustbot label C-enhancement "good first issue"
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by locating the unnecessary_lazy_evaluations lint implementation and its existing tests in rust-clippy. Reproduce the Option::map_or_else and Result::map_or_else examples from the issue, while keeping the stated map_or limitation in mind. Done means both examples receive the intended eager alternative without incorrectly linting map_or.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- tooling
- Issue type
- Feature
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 68/100